The present invention relates to an air/fuel ratio detection apparatus for determining air/fuel ratio of, for example, automotive engines from oxygen concentration or the like of exhaust gas.
In general, automotive engines and the like are provided with an air/fuel ratio sensor (oxygen sensor) in the middle of exhaust pipe or the like for detecting oxygen concentration or the like of exhaust gas.
The air/fuel ratio sensor outputs air/fuel ratio detection signal. Based on this signal, it is possible to conduct a feed-back control of the amount of fuel injection, thereby achieving stoichiometric air/fuel ratio (A/F=14.7) or lean air/fuel ratio (A/F≧15). With this, it is possible to improve engine combustion efficiency and fuel consumption.
There are such conventional air/fuel ratio sensors, which are plate-like in shape (see Japanese Patent Laid-open (Kokai) Publication Showa 61(1986)-10762, Japanese Patent Laid-open (Kokai) Publication Showa 61(1986)-100651, and Japanese Patent Laid-open (Kohyo) Publication Heisei 7(1995)-501152 corresponding to U.S. Pat. No. 5,507,174). These plate-like air-fuel ratio sensors include a heater portion and a solid electrolyte layer and a diffusion layer, which are formed on one major surface of the heater portion.
There are recent demands for the improvement in productivity of air/fuel ratio sensors and for freedom in the direction of installation of air/fuel ratio sensors. With this freedom, it becomes possible to improve the accuracy for detecting oxygen concentration and the like and thereby to properly control the amount of fuel injection and stabilize the engine control. Furthermore, there are recent demands for the rapid temperature increase of the heater portion of air/fuel ratio sensors after the engine starts. With this rapid increase, it becomes possible to shorten the period of time required for activating the solid electrolyte layer and the like, thereby shorten the period of time required for enabling the measurement of oxygen concentration and the like. This makes it possible to properly conduct a feedback control of the amount of fuel injection, based on the signal from air/fuel ratio sensors, at an early stage after the engine starts.